Polysulfone membranes and hollow fiber membranes are known to the art and have been prepared to a variety of specifications. However, prior art hollow fibers typically are asymmetrical, or anisotropic. This type of fiber comprises a "skin" at the inner and/or outer surface and a microporous understructure. For example, U.S. Pat. No. 3,691,068 (Cross) discloses an anisotropic microporous polysulfone polymer membrane having a barrier layer at a surface thereof and a more porous support layer integral with the barrier layer. Similarly, U.S. Pat. No. 4,029,582 (Ishii et al.) discloses poly(arylethersulfone) semipermeable membranes having a thin dense layer and a porous supporting layer.
Skinless polysulfone hollow fibers are known to the art. U.S. Pat. No. 4,612,119 (Eguchi) discloses preparation of a polysulfone hollow fiber filter medium having substantially no skin layer at the outer and inner surface areas. Eguchi uses a dry-wet spinning process, with carefully controlled temperatures. Cabasso et al., "Polysulfone Hollow Fibers II. Morphology," J. Applied Polymer Science, Vol. 21, pp. 165-80 (1977), which also utilizes a dry-wet spinning method, reports that control of the extrusion/coagulation procedure allows the formulation of skinned, porous skinned and non-skinned fibers. In a dry-wet spinning process, the membrane casting solution is extruded into an air space and then is conducted into a liquid precipitation medium.
A major drawback of using membranes for concentration or fractionation of proteins in biotechnology downstream separation processes is the adsorption of protein molecule solutes on the membrane surface. This has an important impact on the flux decline, usually called "membrane fouling," and the performance characteristics of the membrane change with operating time. As a consequence, the separation process design becomes more difficult to optimize and sometimes requires complex operation modes. The easiest way to overcome this problem is to employ a protein non-adsorptive membrane.
U.S. Pat. No. 4,787,976 (Parham) discloses protein non-absorptive filtration membranes which does not include a catalyst in the coagulation solution. U.S. Pat. No. 4,970,034 (Ly) discloses a process for preparing microporous polysulfone membranes. Since there is no catalyst in the Ly coagulation solutions, the membranes formed do not have the increased strength, flux and protein non-adsorptive characteristics of the present invention.